The present invention relates to an apparatus and method and apparatus for designing a cavity for an accelerator and, more particularly, embodiments of the present invention relate to an apparatus and method for establishing the Q-factor of an RF cavity at a desired level.
Particle accelerators have been used for a number of years in various applications. For example, one common and important application of particle accelerators is their use in medical radiation therapy devices. In such an application, an electron gun may be coupled to an input cavity of an accelerator (e.g., a linear accelerator) and provide a source of charged particles or a particle beam to the accelerator. The accelerator may include number of RF (radio frequency) cavities through which charged particles beam travel. Electric and magnetic fields present within the cavity and acting on the charged particles provide the acceleration. The distribution of fields in an RF cavity is primarily determined by the geometry of the RF cavity. The accelerator accelerates the charged particles to produce an accelerated output beam for use in medical radiation therapy. One or more RF cavities in the accelerator are used to couple power into the particle beam to increase its acceleration.
The quality factor (also referred to as Q-factor) of an RF cavity characterizes the quality of the cavity with respect to RF losses in the cavity. The Q-factor of an RF cavity is defined as Q=xcfx89W/Pd, where W is the maximum stored energy in the cavity, xcfx89 is the angular (resonant) frequency of the cavity, and Pd is the power dissipated on the cavity inner wall per radian of the RF cycle. The maximum stored energy W in the cavity is determined by the cavity shape and volume while Pd is determined by the resistivity and magnetic permeability of the material of the inner wall of the cavity.
An RF cavity having a high Q-factor is a more efficient user of RF power. Thus, for the same cavity shape and the same amount of RF power, the accelerating field produced in the cavity is higher in a cavity having a higher Q-factor. However, operational bandwidth of an RF cavity is inversely proportional to the cavities Q-factor. As a result, a cavity having a lower Q-factor can operate on a wider range of frequencies and may be more stable and less sensitive to input perturbations.
It would be advantageous to provide an apparatus and method overcame the drawbacks of the prior art and allowed for an RF cavity that provided a desired Q-factor for the cavity while enabling a desired field distribution for electron acceleration within the cavity.
Embodiments of the present invention provide a method and apparatus for providing an RF cavity with a desired Q-factor and a desired electric and magnetic field distribution within the RF cavity. The cavity may be used in an accelerator, such as a linear accelerator. According to some embodiments of the present invention, different parts of the inner wall of the cavity may be comprised of different materials. For example, one portion of the inner wall of the cavity may be fabricated from or comprise copper while a different portion of the inner wall of the cavity may be fabricated from or comprise steel. Different types of steel having different magnetic permeabilities may create different Q-factors for the cavity. In addition, different positions, ratios or combination of different materials for the inner walls of the cavity may result in different Q-factors for the cavity, even though the electric and magnetic field distributions within the cavity may remain steady. Electric field characteristics and distributions within a cavity, as well as the stored energy in the cavity may be modeled or calculated using SUPERFISH code.
Additional objects, advantages, and novel features of the invention shall be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention.
According to some embodiments of the present invention, a cavity for a linear accelerator may include an inner wall forming a cavity that has an input aperture and an output aperture, wherein the inner wall includes a first portion that includes a first material and a second portion that includes a second material. In some embodiments, an apparatus for a linear accelerator may include a cavity formed, at whole or in part, by a first inner wall portion and a second inner wall portion, wherein the first inner wall portion comprises a first material, the second inner wall portion comprises a second material and the second inner wall portion forms an end plate. In some embodiments, an apparatus for use with a linear accelerator may include a plurality of cavities, wherein at least one of the plurality of cavities is formed, at least in part, by a first inner wall portion and a second inner wall portion, wherein the first inner wall portion comprises copper and the second inner wall portion comprises steel. In some embodiments, an apparatus for use with a linear accelerator may include a portion of a first metallic material; and a portion of a second metallic material coupled to the portion of a first metallic material to form a cavity having an input aperture and an output aperture.
According to some embodiments of the present invention, a method for establishing the Q-factor of a cavity for a linear accelerator may include constructing a cavity having a first inner wall portion and a second inner wall portion, wherein the first inner wall portion includes a first material and the second inner wall portion includes a second material that is different from the first material. In some embodiments, a method for determining configuration of a cavity for a linear accelerator may include determining an internal geometry for a cavity that produces a desired electron acceleration within the cavity when electrons are introduced into the cavity; determining a desired Q-factor for the cavity; and determining a position of a portion of a first material and a position of a portion of a second material for forming the cavity such that the cavity has the desired Q-factor and the internal geometry.
According to some embodiments of the present invention, a system for identifying a configuration of a cavity for a linear accelerator may include means for identifying an internal geometry for a cavity that produces a desired electron acceleration within the cavity when electrons are introduced into the cavity; means for identifying a desired Q-factor for the cavity; and means for identifying a position of a portion of a first material and a position of a portion of a second material for forming the cavity such that the cavity has the desired Q-factor and the internal geometry.
According to some embodiments of the present invention, a computer program product in a computer readable medium for identifying configuration of a cavity for a linear accelerator may include first instructions for identifying an internal geometry for a cavity that produces a desired electron acceleration within the cavity when electrons are introduced into the cavity; second instructions for identifying a desired Q-factor for the cavity; and third instructions for identifying a position of a portion of a first material and a position of a portion of a second material for forming the cavity such that the cavity has the desired Q-factor and the internal geometry.
With these and other advantages and features of the invention that will become hereinafter apparent, the nature of the invention may be more clearly understood by reference to the following detailed description of the invention, the appended claims and to the several drawings attached herein.